Compute the excess kurtosis of a sample. This is a measure of
the "peakedness" of its distribution. A high kurtosis indicates
that more of the sample's variance is due to infrequent severe
deviations, rather than more frequent modest deviations.

Standard deviation. This is simply the square root of the
maximum likelihood estimate of the variance.

Single-pass functions (faster, less safe)

The functions prefixed with the name fast below perform a single
pass over the sample data using Knuth's algorithm. They usually
work well, but see below for caveats. These functions are subject
to array fusion.

Note: in cases where most sample data is close to the sample's
mean, Knuth's algorithm gives inaccurate results due to
catastrophic cancellation.